It is well known that sealing an electronics assembly or system in an enclosure to exclude an ambient environment can improve reliability of the system. Effects on the system of damp, particles, pollution, and changes in ambient atmospheric pressure and density may be substantially eliminated. It is known that for high voltage systems protection from these undesirable environments is even more important and use of a sealed enclosure to protect a high voltage assembly or system is well known.
Known methods, such as described in WO 2005/081401, may also be used to extract heat from an assembly in an enclosure, for example, by circulation of air between an internal heat exchanger and an external heat exchanger.
High voltage breakdown is very difficult to avoid in a low pressure gas environment, so it is not uncommon to fill such an enclosure with a liquid such as mineral oil, silicone oil 561, or a liquid fluorocarbon such as Fluorinert™ Electronic Liquid FC-72 available from 3M™, 3M Center, St. Paul, Minn. 55144-1000, USA. While this enhances both cooling and voltage hold off capabilities of electrical equipment within the enclosure, filling the enclosure with liquid significantly increases a weight of a system and requires means to accommodate expansion of the liquid. This may restrict usable locations or positions of the assembly. Many otherwise suitable liquids have a high dielectric constant and this significantly raises stray capacitance, which can be undesirable in, for example, high voltage Switched Mode Power Supply (SMPS) applications. Moreover, some otherwise suitable liquids are very expensive and this additional cost can have significant adverse implications on a cost of the complete system.
U.S. Pat. No. 4,440,971 discloses the use of unstable supersaturated vapor dielectrics but only a momentary and instantly formed high breakdown strength is possible without refrigeration.